As an acoustic wave device using an acoustic wave, there is known a surface acoustic wave (SAW) device including an interdigital transducer (IDT) composed of a pair of comb-type electrodes formed on a piezoelectric substrate and a pair of reflectors between which the IDT is interposed. The SAW device is used in, for example, a band-pass filter in various circuits that process RF (radio frequency) signals in a frequency band of 45 MHz˜2 GHz, which is typical in cellular phones. Recently, due to the increasing performance of RF communications devices that are typically cellular phones (for example, multiband and multimode), there has been a growing demand for reducing the insertion loss of filters in order to achieve improvement in the receive sensitivity of RF communication devices, power consumption reduction and the like.
Various acoustic wave devices intended to reduce power consumption have been proposed. For example, there is a proposed structure in which the thickness of part of a bus bar is larger than that of electrode fingers (see Japanese Laid-Open Patent Application Nos. 2004-328196 and 2002-100952, for example). There is another proposed structure in which a protection film that covers IDT is thick in positions where electrode fingers are interleaved, as compared to other positions (see Japanese Laid-Open Patent Application No. 2007-110342, for example). There is still another proposed structure in which tips of electrode fingers are widened or a metal film is added to the tips (see Japanese Laid-Open Patent Application No. 2012-186808, for example). There is a further proposed structure in which an in insulator is buried between tips of electrode fingers and those of dummy electrode fingers (see Japanese Laid-Open Patent Application No. 2009-278429, for example). There is a still further proposed structure in which a film having a substance different from that of electrode fingers is provided between adjacent electrode fingers (see Japanese Laid-Open Patent Application No. 7-115342).
However, there is room for improvement in terms of reduction in insertion loss. The proposed structure in which the protection film that covers IDT is thick in positions where the electrode fingers are interleaved, as compared to other positions (see Japanese Laid-Open Patent Application No. 2007-110342) affects performance such as reduction of the electromechanical coupling coefficient. This publication discloses a method for forming the difference in the thickness of the protection film by etching. However, this method is likely to have frequency deviations and has difficulty in manufacturing. Similarly, the structure in which the metal film is added to the tips of the electrode fingers (see Japanese Laid-Open Patent Application No. 2012-186808) has adverse effects on the performance such as reduction of the electromechanical coupling coefficient.